fiend in WordNet English dictionary

1. an evil supernatural being
   devil  fiend  demon  daemon  daimon 
   
2. a cruel wicked and inhuman person
   monster  fiend  devil  demon  ogre 
   
3. a person motivated by irrational enthusiasm (as for a cause)
   "A fanatic is one who can't change his mind and won't change the subject"--Winston Churchill
   fanatic  fiend